Stiffness of the inferior oblique neurofibrovascular bundle

Purpose. To asses the mechanical ability of the inferior oblique neurofibrovascular bundle (NFVB) to act as an ancillary origin for the inferior oblique muscle after anterior transposition. Methods. Stress-strain relations and Young's modulus of elasticity, a measure of tissue stiffness, were determined for the NFVB in vitro, in situ, and in vivo in dynamic and static conditions. For comparison, similar studies were performed in vitro and on the superior oblique tendon (SOT). Results. Young's moduli for NFVB in situ (6.3 MPa [megapascals] and in vivo (11.8 MPa) were approximately 2 and 4 times greater (P < 0.05), respectively, than those of isolated NFVB in vitro at 5% to 10% dynamic strain (3 MPa). In dynamic conditions, Young's moduli in vitro for the NFVB and the SOT were similar. Conclusions. The NFVB is a biomaterial that has stiffness properties similar to the SOT. Within the range of forces typical of normal eye movements (79 to 393 mN), the NFVB alone can tolerate forces of 98 mN at 0% to 10% strain and 393 mN at 15% to 20% strain, based on dynamic in vitro analysis. The greater measured stiffness in situ and in vivo suggest that the NFVB in the intact orbit potentially has a resting strain of 15% to 20%, and additional tissues in parallel with the NFVB also contribute to total stiffness. These data support the hypothesis that the NFVB, acting alone or in concert with adjacent orbital tissues, may form an ancillary origin for the inferior oblique muscle after anterior transposition.